THIS phenomenon is often observed, though not often so clearly as in the case noticed by Mr. Poulton. It is due to the same cause as produces the dust-free space seen rising from hot bodies in illuminated smoky air, viz, a peculiar Crookesian (or rather Osborne Reynoidsian) bombardment of sufficiently small dust-particles, in the direction of decreasing temperature, by the extra energy of the gas-molecules on one side. See papers by myself and the late Mr. Clark in NATURE (especially July 26, 1883, April 24, 1884, vol. xxix. p. 417, and January 22, 1885), and in Phil. Mag, 1884, Proc. R.I., c also by Mr. Aitken, Trans. R.S. Edin., 1884. And see the remarkable theoretical paper by Prof. Osborne Reynolds on “Dimensional Properties of Gases,” Phil. Trans., 1879.